[PATCH] Speculatively instantiate archive members

[Simon Atanasyan]

================
Comment at: lib/ReaderWriter/FileArchive.cpp:102
@@ +101,3 @@
+    group.spawn([this, promise, ci, &group] {
+      std::unique_ptr<File> result;
+      if (instantiateMember(ci, result)) {
----------------
It looks like we can remove the `group` from the capture list because it is not used in the lambda body.

================
Comment at: lib/ReaderWriter/FileArchive.cpp:274
@@ +273,3 @@
+  mutable std::map<const char *, std::future<const File *>> _preloaded;
+  mutable std::vector<std::unique_ptr<std::promise<const File *>>> _promises;
+  mutable std::mutex _mutex;
----------------
Can we store promises directly and do not use `unique_ptr` wrapper? Though I did not test the following code, it compiled successfully. Does it have a sense?

```
// Instantiate the member
_promises.emplace_back();
auto &promise = _promises.back();
_preloaded[memberStart] = promise.get_future();
                                               
group.spawn([this, &promise, ci] {
  std::unique_ptr<File> result;
  if (instantiateMember(ci, result)) {
    promise.set_value(nullptr);
    return;
  }
  promise.set_value(result.release());
});

[...]

mutable std::vector<std::promise<const File *>> _promises;
```

http://reviews.llvm.org/D7015

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